Lysozyme has been studied in insects as part of the system of inducible antibacterial defence in the haemolymph. We recently found two Drosophila lysozyme genes that are constitutively expressed in the digestive tract, and are probably involved in the digestion of bacteria in the food. To obtain an overview of the lysozyme genes in this species and their possible roles in immunity and digestion, we have now characterized all six lysozyme genes in the cloned part of the lysozyme locus at 61F, and a seventh gene that maps to the same chromosomal location. The expression of the genes follows four different patterns: firstly, four closely related genes, LysB, C, D and E, are all strongly expressed in the midgut of larvae and adults; secondly, LysP is expressed in the adult salivary gland; thirdly, LysS is expressed mainly in the gastric caecae of larvae; and finally, LysX is primarily expressed in the metamorphosing midgut of late larvae and early pupae. The LysD-like genes and LysS are strongly repressed in artificially infected animals, possibly reflecting a malaise reaction in the digestive tract. None of the genes is expressed in the fat body or haemocytes. Thus rather than being a component of the haemolymph, the Drosophila lysozymes are found mainly in the digestive tract where they are expressed at a high level. Furthermore all genes, except LysP, encode acidic proteins, in contrast to the strongly basic "typical" lysozymes. This is highly reminiscent of the situation in ruminants, where the lysozymes have been recruited for the digestion of symbiotic bacteria in the stomach.

Cecropins are antibacterial peptides that are synthesized in insects as a response to infection. As a first step towards a molecular study of the induction of this response, we have isolated genomic clones that cover the cecropin locus in Drosophila melanogaster. This locus was found to be unique, and it was mapped cytologically to the chromosomal location 99E. Sequence analysis showed it to be unusually compact, with three expressed genes and two pseudogenes within less than 4 kb of DNA, and with another homologous region less than 4 kb away. Two of the genes, A1 and A2, encode a product that is identical to the major cecropin from Sarcophaga peregrina, while the cecropin encoded by the B gene differs in five positions. Cecropin transcripts appear within an hour after bacteria have been injected into the hemocoel, reach a maximum after 2‐6 h, and have almost disappeared again after 24 h. The B gene is induced in parallel with the A genes, but on a lower level. The cecropin genes were also induced when the flies were kept on food with the Drosophila pathogenic bacterium Serratia marcescens Db10 or its non‐pathogenic derivative Db1140.

Cecropins are antibacterial peptides, induced in Drosophila as part of the humoral immune response to a bacterial invasion. We have used the cloned Drosophila cecropin genes CecA1, A2 and B as probes to study the developmental and tissue specific regulation of this response. The genes are strongly expressed in fat body and hemocytes after injection of bacteria, the CecA genes being much more active than CecB in the fat body. All parts of the fat body and 5‐10% of the hemocytes are involved in this response. CecA1 and A2 are most active in larvae and adults; CecB is preferentially active in early pupae. A small peak of constitutive cecropin expression in early pupae appears to be caused by bacteria in the food. Cecropin A, the common product of the CecA1 and A2 genes, was identified in the hemolymph of immunized flies at a concentration of 25‐50 microM, enough to kill all tested bacteria except Serratia, a Drosophila pathogen. A useful in vitro system to study the immune response has been found in Schneider's line 2 cells which respond to lipopolysaccharide and laminarin by cecropin expression.

In our study of the cecropin locus in Drosophila we have found a gene for a new peptide, andropin, with antibacterial properties. Transcripts from this gene, Anp, could be detected in newly eclosed males and reached steady‐state levels after 1 day. Transcription was strongly induced in response to mating and is strictly confined to the ejaculatory duct of adult males. The deduced peptide sequence reveals a hydrophobic amino terminus with striking similarity to the signal peptide of the cecropins. The sequence of the predicted mature andropin shows no direct homology with the cecropins, but the two peptides may have similar secondary structures. We have synthesized the predicted gene product and shown it to be antibacterial. Crude extracts from male genital tracts show a potent bactericidal activity, and electrophoretic separation revealed at least three antibacterial components, one with the same mobility as the synthetic peptide. It appears that insects have evolved a mechanism for the protection of the seminal fluid and the male reproductive tract against microbial infections.

Epidermal growth factor receptor (EGFr) is a key mediator of cell communication during animal development and homeostasis. In Drosophila, the signaling event is commonly regulated by the polytopic membrane protein Rhomboid (RHO), which mediates the proteolytic activation of EGFr ligands, allowing the secretion of the active signal. Until very recently, the biochemical function of RHO had remained elusive. It is now believed that Drosophila RHO is the founder member of a previously undescribed family of serine proteases, and that it could be directly responsible for the unusual, intramembranous cleavage of EGFr ligands. Here we show that the function of RHO is conserved in Gram-negative bacteria. AarA, a Providencia stuartii RHO-related protein, is active in Drosophila on the fly EGFr ligands. Vice versa, Drosophila RHO-1 can effectively rescue the bacterium's ability to produce or release the signal that activates density-dependent gene regulation (or quorum sensing). This study provides the first evidence that prokaryotic and eukaryotic RHOs could have a conserved role in cell communication and that their biochemical properties could be more similar than previously anticipated.